There is a considerable commercial interest in SMA tubing, which has many technical uses, including many uses in medical implants or medical device components. The SMA of prime interest for the present invention is a nickel-titanium alloy, generically known as “Nitinol” (or NiTi). Throughout the present description and claims this single name will be used, but it is to be understood that included within the invention are all the binary, tertiary or more complex nickel-titanium alloys having desirable shape memory properties. That is, the application of the invention is not limited to any specific NiTi SMA formulation.
In the formation and size reduction of tubing, one method is to draw the tubing through a series of conical converging dies, in combination with a series of mandrels inserted in the tube internal diameter (“ID”), in order to achieve decreasing tubing outer and inner diameters. Most commonly, the dies are made from a hard material, such as a metal carbide or synthetic or natural diamond. However, the combination of such dies and a oxide free NiTi surface can cause a phenomenon known as “galling”, or surface erosion and scratching, due to the particular affinity of the Ti in the NiTi for the carbon in the metal carbide or diamond dies. In extreme cases, galling can cause destruction of an expensive die and process interruption.
The Nitinol industry has developed methodology to minimise or overcome the problem of galling. This uses oxidation of the surface of the Nitinol to form a surface layer of TiO2, of thickness in the range of, usually, 600 to 3000 Angstrom (60 to 300 nanometre), which combined with conventional drawing lubricants, eliminates galling and results in successful diameter reduction and a reasonable die lifetime. The oxide may be formed by heat treating in an atmosphere that contains oxygen such as air or a controlled combination of inert gas with ppm levels of oxygen in the range of 50-1000 ppm. The rapid strain hardening rate and commensurate decrease in ductility of NiTi requires frequent annealing during the tube manufacturing process. It is desirable that the annealing and surface oxidation treatments be accomplished simultaneously. When performing the annealing heat treatment in air, the rate of oxidation may be so rapid that an excessive surface oxide forms. Therefore a controlled oxygen level created by blending ppm levels of oxygen with inert gases is preferred. In this manner, temperatures required for annealing may be attained without excessive oxidation.
The very rapid strain hardening rate and commensurate decrease in ductility of Nitinol does not readily permit the formation of seamless tube by the deep draw method. The preferred method is to drill a central hole in a centerless ground rod to produce the desired wall thickness or OD/ID ratio (outside diameter/internal diameter). The centerless ground OD and drilled/honed ID provide optimum surface conditions for initiating the tube drawing process.